1. Description of the Prior Art
The present invention is a system for transmitting multiple communication signals, including at least one video signal, simultaneously over a link formed of a twisted pair of telephone wires.
2. Field of the Invention
In conventional telecommunication systems the transmission of full motion video images has heretofore been possible only over communication links especially adapted for broadband transmission, as contrasted with the bandwidth of signals employed in the transmission of telephone communications. The transmission of voice signals in telephone communications requires a bandwidth of about 4 kilohertz. This bandwidth will accommodate frequencies within the range of the human voice and the frequencies of dial and pulse code tones used for establishing connections.
To meet the requirement for providing telephone service to communities throughout this country and throughout the world an infrastructure of telephone lines has been established from telephone central offices to subscriber locations. In many countries including this country such telephone lines are formed of lengths of twisted pairs of insulated copper wires of 16, 22, 24 or 26 gauge (AWG). Incalculable lengths of such twisted pairs of telephone wires have been installed by suspension from telephone poles and in buried conduits to link subscriber locations to telephone central offices, which in turn are linked throughout the country and throughout the world by telephone communications networks.
Due to the impedance of a link formed of a twisted pair of telephone wires there is always an attenuation of the electrical communication signals transmitted over these wires. The extent of this attenuation depends upon the length of the twisted pair link. That is, the longer the length of the twisted pair link the greater will be the total impedance between the central office and the subscriber's handset.
It is desireable in conventional telephone systems to create a uniformity in the attenuation of a signal from a subscriber location so that all of the telephone signals reaching a central office can be uniformly amplified and processed through the same type of common carrier equipment. The attenuation in a conventional link formed of a twisted pair of telephone wires below a prescribed frequency can be reduced and the lost frequency characteristic made nearly flat, by inserting series inductance periodically. This practice of inserting inductance periodically is termed loading. It has been conventional practice to load twisted pairs of telephone wires at intervals of six thousand feet where the wires are designed to conduct frequencies of from 0 to 4 kilohertz. The loading coils are normally located in manholes and in elevated equipment boxes on telephone poles. The conventional loading practice provides a relatively uniform signal level at telephone central offices from subscriber locations located at varying distances from the central offices.
While conventional telephone lines formed of twisted pairs of wire are suitable for the transmission of voice communications between telephone handsets and also for data communications, such as between computers and facsimile machines, the transmission of video signals over conventional telephone lines including twisted pair links has heretofore been impractical. Unlike voice and data communications which require a bandwidth of only about 4 kilohertz, the transmission of full motion video signals, such as television signals, requires a bandwidth of a minimum of 4.5 megahertz. In telephone communications systems in this country the Federal Communications Commission requires telephone communication carriers to allocate a bandwidth of 6 megahertz for the transmission of television signals. In other countries, such as in Europe, a bandwidth of 7.5 megahertz is required.
The reason that a video signal requires a bandwidth far greater than a voice signal is because of the multitude of points which must be scanned in a horizontal scanning raster in order to convey a video picture. At the end of each scan a blanking pulse is generated with respect to the video information to be reproduced, and a line synchronizing pulse of a polarity opposite to the video information is generated during this blanking pulse. The line pulses synchronize the individual horizontal scanning lines. Similarly, it is necessary to synchronize the field or vertical scans. This is done by another pulse train which occurs during the field retrace time.
In conventional full motion or "fast scan" television transmission, the time interval from the start of one horizontal line to the next is 63.5 microseconds. Each complete frame consists of two interlaced fields. The frame rate is 30 per second and there are 525 scanning lines per frame, of which about 93% are visible because of the loss of time during the field blanking pulse. To make a horizontal resolution approximately equal to a vertical resolution, a minimum bandwidth of about 4.5 megahertz is required. This frequency range and above will hereinafter be referred to as a broadband frequency spectrum, as contrasted with the narrow band frequency spectrum of 4 kilohertz which is employed in voice and data transmission.
Conventional lines currently employed in the infrastructure of telephone communications have heretofore been considered unsatisfactory for the transmission of video signals due to the degradation which video signals experience in passing over a conventional twisted pair of telephone wires. For a length of more than about one thousand feet, the horizontal sync pulses of a video signal lose their definition and become intermixed with the video data. As a consequence, the receiver cannot distinguish between horizontal scan lines. Also, the portions of the signals near the upper and lower ends of the frequency band experience far greater degradation than the portions of the signals near the middle of the band. Since color information is transmitted at the upper end of the frequency band of a video signal, color quickly becomes lost in transmission of a video signal over links including conventional twisted pairs of telephone wires. Thus, the transmission of full motion televised video signals, especially color video, has heretofore required broadband carrier facilities. The existing infrastructure of telephone communications lines has heretofore been considered unacceptable for the transmission of full motion video signals.